Corrosion is the disintegration of a base material as a result of chemical reactions with the surrounding environment(s) and generally refers to the electrochemical oxidation of metals resulting from contact with an oxidant such as oxygen or chlorine. Given the importance of metals in manufacturing and the exposure of the manufactured articles to a range of corrosive environments, methods and materials for controlling or suppressing corrosion are of continued interest in many industries.
Rusting of an iron or steel substrate is an electrochemical process that begins with the transfer of electrons from iron to oxygen, the rate of corrosion being affected by a number of factors including the presence of water and any electrolytes. The key reaction is the reduction of oxygen according to Reaction I:O2+4e−+2H2O→4OH−  (I)
Because it forms hydroxide ions, this process is strongly affected by the presence of acid. And, indeed, the corrosion of most metals is accelerated under lower pH conditions. Providing the electrons for Reaction I is the oxidation of iron that may be described as follows:Fe→Fe2++2e−  (II)
The redox reaction illustrated in Reaction III also occurs in the presence of water and is crucial to the formation of rust:4Fe2++O2→4Fe3++2O2−  (III)
Additionally, the following multistep acid-base reactions as illustrated in Reactions IV and V can affect the rate of rust formation:Fe2++2H2OFe(OH)2+2H+  (IV)Fe3++3H2OFe(OH)3+3H+  (V)as do the dehydration equilibria illustrated in Reactions VI-VIII:Fe(OH)2FeO+H2O  (VI)Fe(OH)3FeO(OH)+H2O  (VII)2FeO(OH)Fe2O3+H2O  (VIII)
From the reactions detailed above, it may be appreciated that the corrosion products are dictated in large part by the availability of both water and oxygen. Accordingly, in those instances with limited dissolved oxygen, the formation of iron (II)-containing compounds will be favored including, for example, FeO and black lodestone (Fe3O4). Higher oxygen concentrations tend to favor the formation of ferric materials that generally fall within a nominal formula that can be expressed as Fe(OH)3-xOx/2. Furthermore, these complex “rusting” reactions will be affected by the presence of other ions including, for example, Ca2+, which can serve a double role as both an electrolyte, which tends to accelerate rust formation, and as a reactant species capable of combining with the hydroxides and oxides of iron to form precipitates comprising a range of Ca—Fe—O—OH species.
One method of protecting metals from corrosion involves forming a barrier coating in order to separate the metal from the surrounding and potentially corrosive environment. Examples of such barrier coatings include paints and nickel and chrome plating. Paints can be problematic for those components that will be subsequently subjected to one or more high temperature processes including, for example, welding and/or heat treating. Further, as with all barrier coatings, defects in or damage to the barrier coatings leave the underlying metal substrate susceptible to corrosion. Further, electrochemically active barrier coatings including, for example, nickel, chrome, and conductive polymer layers, can actually accelerate corrosion of underlying metals once an opening is formed in the coating.
Other coatings used to protect metal substrates include sacrificial coatings in which the coating material(s) react with the environment and is consumed while leaving the underlying substrate substantially intact. These sacrificial coatings may be subdivided into chemically reactive coatings including, for example, chromate coatings, and electrochemically or galvanically active coatings including, for example, aluminum, cadmium, magnesium, zinc and combinations thereof. The galvanically active coatings must be conductive and are commonly referred to as “cathodic” protection.
In the art, a major difficulty has been the creation of a coating that protects like a cathodic system but is applied with the ease of a typical barrier coating system. Furthermore, there are many environmental drawbacks associated with traditional barrier and sacrificial methods including, for example, high levels of volatile organic compounds, toxic or suspect compounds and/or expensive waste treatment and environmental requirements.
The present invention contemplates an improved anti-corrosion coatings and methods of forming such coatings which address some of the limitations and concerns associated with conventional coating methods while providing improved coating performance.